The present invention relates generally to a telephoto lens system, and more specifically to a telephoto lens system having an angle of field of about 8.24.degree., which is well suited for a large-aperture lens.
In many proposals made so far of telephoto lenses, the rear lens unit is often designed to be movable during focusing while weight balance during shooting, i.e., manipulability is taken into account. This design is particularly effective for super-telephoto lenses of increasing size. For instance, JP-A 50(1975)-139732 and 51(1976)-78326 achieve manipulability improvements by designing a negative lens group of the rear lens unit to be moved toward the image surface side. On the other hand, JP-A 52(1977)-117126 achieves similar effects by designing a positive lens group of the rear lens unit to be moved toward the object side.
It is JP-A 53(1978)-134425 and 59(1984)-36218 that disclose that such design is far more effective for a large-aperture telephoto lens system of increasing size.
As regards optical performance or quality, however, it is known that a focusing system making use of the movement of the rear lens unit is smaller in the amount of focusing movement at the same object distance than that making use of the movement of the overall system or front lens unit, but this system incurs some considerable aberration variations incidental to focusing.
A typical example of lens design for eliminating spherical aberration variations is disclosed in JP-A 55(1980)-147606 teaching that the spherical aberration variations are eliminated by a lens arrangement including a focusing lens unit together with a lens unit of very low refracting power. However, this lens design is less practical for shooting a nearby object.
Hereupon, it is known that a telephoto lens system comprising a front lens unit of positive refracting power and a rear lens unit of negative refracting power and having a reduced telephoto ratio can be achieved by placing the refracting power profile of the two lenses under properly selected conditions, and that the chromatic aberration peculiar to a telephoto lens system can be eliminated by use of anomalously partial-dispersing glass.
However, a grave technical problem remains unsolved about correction of noticeable aberration variations due to focusing by the rear lens unit when a shot is taken of a nearby object.